NO.sub.x emissions from gas flames can be created either through the Zeldevitch mechanism (often called thermal NO.sub.x) or through the formation of HCN and/or NH.sub.3 which can then be ultimately oxidized to NO.sub.x (prompt NO.sub.x). Thermodynamic calculations typically show that NO.sub.x emissions measured from natural gas flames are well below, one to two orders of magnitude, the thermodynamic equilibrium value. This indicates that in most situations NO.sub.x formation is kinetically controlled.
Kinetic calculations indicate that thermal NO.sub.x emissions are typically the most important source of NO.sub.x for natural gas flames, with the NO.sub.x being created through the following reactions: EQU N+O.sub.2 =NO+O (1) EQU N+OH=NO+H (2) EQU N.sub.2 +O=NO+N (3)
Kinetic calculations were performed using a PC version of the CHEMKIN computer program. Calculations using this program have provided valuable insight into changes in the burner fuel and air mixing characteristics which can lower NO.sub.x emissions.
As the name implies, thermal NO.sub.x can be controlled by regulation of the peak flame temperature, and as shown in FIG. 1 using kinetic calculations, if the temperature can be lowered enough the NO.sub.x emissions from a "true" premixed natural gas flame operating at 15% excess air can be reduced to extremely low values (less than 1 ppmv). In effect FIG. 1 shows the relationship between thermal NO.sub.x and temperature since for a premixed natural gas flame with an excess of oxygen, thermal NO.sub.x is the only route by which any significant NO.sub.x emissions are created.
Under appropriate flame conditions the formation of prompt NO.sub.x can also be important when burning natural gas. The kinetic model used shows that under fuel rich conditions, particularly when the stoichiometry is under about 0.6, both HCN and NH.sub.3 can be formed through reaction of CH with N.sub.2 to form HCN and N. These calculations were conducted using gas and air mixtures with stoichiometries ranging from 1.0 to 0.4. The model predicts that prompt NO.sub.x becomes important at higher stoichiometries when the temperature is lower; see FIG. 2. Below a stoichiometry of 0.5 almost all the NO.sub.x formed is prompt NO.sub.x. The rate of prompt NO.sub.x formation (as the name implies) is also very rapid, being nearly complete in about 1 millisecond at a temperature of 2400.degree. F.
Kinetic calculations also indicate that hydrocarbon fragments, in addition to being important for prompt NO.sub.x, are also important for thermal NO.sub.x formation since they can act as a source of O atoms and OH radicals. Kinetic calculations show the importance of the hydrocarbon concentration in the formation of NO.sub.x, even under oxidizing conditions. At a temperature of 3400.degree. F. the predicted NO.sub.x emissions were about 4 ppmv after 5 ms residence time for a mixture of N.sub.2, O.sub.2, H.sub.2 O, and CO.sub.2 when hydrocarbons were not present, as compared to 80 ppmv when combustion of about 1% CH.sub.4 was present in the gas mixture. If the concentration of methane initially present was reduced to about 0.5%, the NO.sub.x concentration after 5 ms was reduced to about 75 ppmv. The kinetic model used predicts that the following mechanisms are important:
1. Reaction of CH.sub.4 with O.sub.2, OH and H to form CH.sub.3 PA0 2. Reaction of CH.sub.3 with O.sub.2 to form CH.sub.3 O and O PA0 3. Reaction of N.sub.2 with O to form NO and O PA0 4. Various reactions to form OH PA0 5. Reaction of N.sub.2 with OH to form NO and NH
Low NO.sub.x gas burners have been undergoing considerable development in recent years as governmental regulations have required burner manufacturers to comply with lower and lower NO.sub.x limits. Most of the existing low NO.sub.x gas burner designs are nozzle mix designs. In this approach the fuel is mixed with the air immediately downstream of the burner throat. These designs attempt to reduce NO.sub.x emissions by delaying the fuel and air mixing through some form of either air staging or fuel staging combined with flue gas recirculation ("FGR"). Delayed mixing can be effective in reducing both flame temperature and oxygen availability and consequently in providing a degree of thermal NO.sub.x control. However, delayed mixing burners are not effective in reducing prompt NO.sub.x emissions and can actually exacerbate prompt NO.sub.x emissions. Delayed mixing burners can also lead to increased emissions of CO and total hydrocarbons. Stability problems often exist with delayed mixing burners which limit the amount of FGR which can be injected into the flame zone. Typical FGR levels at which current burners operate are at a ratio of around 20% recirculated flue gas relative to the total stack gas flow.
A further type of low NO.sub.x burner which has been developed in recent years is the premixed type burner. In this approach, the fuel gas and oxidant gases are mixed well upstream of the burner throat, e.g. at or prior to the windbox. These burners can be effective in reducing both thermal and prompt NO.sub.x emissions. However, problems with premixed type burners include difficulty in applying high air preheat, concerns about flashback and explosions, and difficulties in applying the concept to duel fuel burners. Premix burners also typically have stability problems at high FGR rates.
In the inventions of my Ser. Nos. 092,979 and 188,586 applications (the disclosures of which are hereby incorporated by reference) extremely low NO.sub.x, CO and hydrocarbon emissions are achieved, while maintaining the desirable features of a nozzle mix burner. This is accomplished by injecting the fuel gas, such as natural gas, in a position that would be typical for a nozzle mix burner, while generating such rapid mixing that, effectively, premixed conditions are created upstream of the ignition point.
In such burner apparatus an outer shell is provided which includes a windbox and a constricted tubular section in fluid communication therewith. A generally cylindrical body is mounted in the shell, coaxially with and spaced inwardly from the tubular section so that an annular flow channel or throat is defined between the body and the inner wall of the tubular section. Oxidant gases are flowed under pressure from the windbox to the throat, and exit from a downstream outlet end. A divergent quarl is adjoined to the outlet end of the throat and define a combustion zone for the burner. A plurality of curved axial swirl vanes are mounted in the annular flow channel to impart swirl to the oxidant gases flowing downstream in the throat. Fuel gas injector means are provided in the annular flow channel proximate or contiguous to the swirl vanes for injecting the fuel gas into the flow of oxidant gases at a point upstream of the outlet end. The fuel gas injection means comprise a plurality of spaced gas injectors, each being defined by a gas ejection hole and means to feed the gas thereto. The ratio of the number of gas ejection holes to the projected (i.e. transverse cross-sectional) area of the annular flow channel which is fed fuel gas by the injector means is at least 200/ft.sup.2.
One or more turbulence enhancing means may optionally be mounted in the throat at at least one of the upstream or downstream sides of the swirl vanes. These serve to induce fine scale turbulence into the flow to promote microscale mixing of the oxidant and fuel gases prior to combustion at the quarl.
The gas injectors can be located at the leading or trailing edges of the swirl vanes, and inject the fuel gas in the direction of the tangential component of the flow imparted by the swirl vanes. The gas injectors can also be disposed on a plurality of hollow concentric rings which are mounted in the throat downstream of the swirl vanes. The injectors can similarly comprise openings disposed in opposed concentric bands on the walls which define the inner and outer radii of the annular flow channel. The gas injectors can also be located at the surfaces of the swirl vanes, with the vanes being hollow structures fed by a suitable manifold. Preferably the geometry of the burner is such that the product of the swirl number S and the quarl outlet to inlet diameter ratio C/B is in the range of 1.0 to 3.0.
Pursuant to another aspect of the Ser. No. 092,979 and Ser. No. 188,586 invention, a method is provided for injection of gaseous fuel in a forced draft burner of the type which includes an annular throat of outer diameter B, having an inlet connected to receive a forced flow of air and recirculated flue gases, and an outlet adjoined to a divergent quarl. The gaseous fuel is injected at an axial coordinate which is spaced less than B in the upstream direction from the axial coordinate at which the quarl divergence begins; and sufficient mixing of the gaseous fuel with the air and recirculated flue gases is provided that these components are well-mixed down to a molecular scale at the axial coordinate of ignition. This procedure results in extremely low NO.sub.x, CO and hydrocarbon emissions from the burner.
In a further aspect of the Ser. No. 092,979 and Ser. No. 188,586 invention, the swirl vanes, which are mounted with their leading edges parallel to the axial flow of fuel and oxidant gases, and then slowly curve to the final desired angle, have a constant radius of curvature along the curved portion of the vane, whereby the curved portion is a section of a cylinder. This shape simplifies manufacturing using conventional metal fabricating techniques.
Additional background which will be helpful in understanding the present invention can be gained by reviewing FIGS. 3, 4, 5 and 6 herein, which describe a representative embodiment of the apparatus disclosed in my prior applications. In FIG. 3 an isometric perspective view thus appears of such prior art embodiment of burner apparatus 51. This Figure may be considered simultaneously with FIGS. 4, 5 and 6, which are respectively longitudinal cross-sectional; and front and rear end views of apparatus 51.
In burner apparatus 51 combustion air (which can be mixed with recirculated flue gas) is provided to the windbox 53 through a cylindrical conduit 55. Windbox 53 adjoins a tubular section 57 which terminates at a flange 59, which is secured to a divergent quarl 58 (FIG. 12). In the arrangement shown, the inner co-axial cylindrical body 61 is comprised of a central hollow cylindrical tube 63 intended for receipt of an oil gun or a sight glass, and a surrounding tubular member or cylinder 65 which is spaced from the outside wall of tube 63 and closed at each end, by closures 67. A hollow annular space 68 is thereby formed between tubular member 63 and cylinder 65, which serves as a manifold 68 for the fuel gas which is provided to such space via connector 69. The cylindrical body 61 is positioned and spaced within wind box 53 and tubular section 61 by passing through flanges, one of which is seen at 71. The latter is secured to a plate 73 at the end of the wind box by bolts 75 and suitable fasteners (not shown). This arrangement enables easy disassembly, as for servicing and the like.
In the arrangement of burner 51, a series of swirl vanes 77 are provided in the annular space or throat 79 which is defined between tubular body 61 (specifically, between the outer wall of cylinder 65) and the inner wall of tubular member 57.) At the immediately upstream end of each of the swirl vanes 77, gas injector means are provided which take the form of a plurality of tubes 81, each of which is provided with multiple holes 83. It will be evident that the tubes 81, being hollow members, are in communication at their open one end with the interior of the gas manifold 68 defined within member 65, which therefore serves as a feed source for the fuel gas. The fuel gas is discharged in the direction of the openings 83, so that in each instance fuel is injected into the throat directly at the leading edges of the swirl vanes and in the direction of the tangential component of the flow imparted by the swirl vanes 77. Accordingly, the gas injection also acts to enhance the swirl number of the flow.
Although the invention of my Ser. Nos. 092,979 and 188,586 applications (hereinafter at times referred to as the "basic rapid mix burner" or "basic RMB") is extremely effective in achieving the desired results, the basic RMB design results in a burner size that is significantly larger than many existing burners. Although the large burner size is not inherently important to the rapid mix feature, the large burner size is important for creating an extremely stable flame which allows high flue gas recirculation rates to be used without concerns about the flame becoming unstable.
Another limitation of the basic RMB design is that the burner geometry must be kept circular. This is clearly a limitation in a boiler or furnace that use square, rectangular or other shape burners.
When the basic RMB is retrofit into existing furnaces, the larger size, relative to the existing burner, can create significant difficulties and increase the retrofit cost. Problems with the larger burner size are particularly apparent when the boiler or furnace burner wall is a "water wall" consisting of pressurized steam or water tubes. For this type design the burner openings are made by bending the boiler tubes. Any significant increase in the burner size entails bending new tubes to make a larger opening. Utility and large field erected industrial boilers typically have the burners inserted through a water wall.
One method of reducing the burner size is to increase the velocities through the burner. However this method has the disadvantage of increasing the pressure drop through the burner. A higher pressure drop through the burner creates other retrofit difficulties, including replacement of forced draft fans, increased operating costs associated with the higher fan pressure, structural limitations on the windbox and increased operating costs.